1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having cogs spaced at regular intervals along the length of the belt.
2. Background Art
Cogged V-belts are well known in the prior art. The cogged V-belt drives/is driven by reason of frictional forces developed between the laterally oppositely facing side surfaces thereof and the facing surfaces on a cooperating V-grooved pulley. The cogged construction gives the belt considerable flexibility to thereby minimize power loss and frictional heat generation resulting from flexing of the belt in operation. Thus, fatigue due to continuous flexing is reduced which results in extension of the useful life of the belt.
The cogged V-belt has a wide field of application. The cogged construction is used, for example, on variable speed belts, belts for driving automotive accessories, belts for driving agricultural machinery, etc.
One drawback with the cogged V-belt is that it generates a harsh percussion noise in use. This noise develops as each cog engages with and disengages from a cooperating pulley. This noise is identified in the industry as "pitch noise".
A greater "pitch noise" is generated between the cogged belt and a driving pulley than between the cogged belt and a driven pulley. Also, a more significant "pitch noise" is developed as the belt engages a pulley than when it disengages therefrom.
More particularly, the "pitch noise" is generated as follows. As the cogs enter a pulley and contact the laterally spaced surfaces bounding the pulley groove, the cogs, in succession, become momentarily stressed, thereby continuously generating the harsh noise. The space between the cogs allows the impact sound to reverberate. This is particularly a problem with V-belts travelling at high speeds, under which conditions the cogs are driven more frequently and forcibly into contact with the pulley surfaces to produce an even harsher, high frequency noise.
Various attempts have been made in the prior art to prevent the "pitch noise" that results from operation of conventional cogged V-belts. One such proposed solution is described in Japanese utility model publication 57-24997 (1982). The cogged V-belt therein has lateral notches on the inside surface of the cogs. As a result, the pitch and height of the cogs are random. While the notches account for a significant suppression of "pitch noise", the notches at the same time weaken the cogs. The cogs tend to crack at the notches, which shortens the life of the belt.
Another proposed solution to the "pitch noise" problem is described in Japanese Utility Model Application Laid-Open 54-95663 (1979). The belt disclosed therein is a V-belt with cogs having a V-shaped cross section. The cogs are formed in a rubber layer having a hardness less than that of the rubber in the compression layer. While a softer rubber accounts for less noise generation, the softer rubber wears out and deforms more readily in use, thereby shortening the useful life of the belt.